1. Field of the Invention
The present invention relates to an organometallic complex capable of being used in an organic electroluminescent device, and more particularly, to a binuclear organometallic complex capable of emitting light with a wide spectrum from a blue region to a red region through triplet metal-to-ligand charge transfer (MLCT) and to an organic electroluminescent device using the same as an organic layer forming material.
2. Description of the Related Art
An organic electroluminescent (EL) device is a spontaneous light-emitting display device that generates light through the recombination of electrons and holes when an electric field is applied to thin films composed of fluorescent or phosphorescent organic compounds (organic layers). An organic EL device has various advantages including lightness, simple components and a simple manufacturing process, high resolution, wide viewing angle, high color purity, perfect reproduction of motion pictures, low power consumption, low driving voltage, and so on so that is suitable for use in portable electronic devices.
An electroluminescent layer of an organic EL device can be composed of a fluorescent material or a phosphorescent material. The fluorescent material differs from the phosphorescent material in terms of a light-emitting mechanism. The fluorescent material uses singlet excitons, and the phosphorescent material uses triplet excitons. An electroluminescent layer is composed of the fluorescent material or the phosphorescent material itself. Alternatively, the fluorescent material or the phosphorescent material is doped into an appropriate host material. In the latter case, as a result of electron excitation, singlet excitons and triplet excitons are produced in the host. Statistically, the singlet excitons and the triplet excitons are produced in a ratio of about 1:3.
A conventional organic EL devices including an electroluminescent layer composed of a fluorescent material is disadvantageous since triplet excitons are consumed by the host. However, a conventional organic EL device including an electroluminescent layer composed of a phosphorescent material are advantageous since singlet excitons and triplet excitons are both utilized to achieve an internal quantum efficiency of 100%. Thus, an organic EL device including an electroluminescent layer composed of a phosphorescent material has higher emission efficiency than an organic EL device including an electroluminescent layer composed of a fluorescent material.
The introduction of a heavy metal such as Ir, Pt, Rh, or Pd to organic molecules has led to spin-orbital coupling due to a heavy atom effect which allows a triplet state and a singlet state to coexist, thus enabling a forbidden transition, thereby allowing phospholuminescence to occur even at room temperature.
Recent developments have led to the discovery of highly efficient green and red luminescent materials using photoelectroluminescence with the internal quantum efficiency as high as 100%. In particular, a green phosphorescent material including fac-tris(2-phenylpyridine)iridium(Ir(ppy)3) has an external quantum efficiency of 17.6±0.5%. {Bis(2-(2′-benzo[4,5-A]thienyl)pyridinato-N,C)iridium(acetylacetonate)} (Btp2Ir(acac)) has been developed as a red phosphorescent material having a high emission efficiency of 7.0±0.5%.
As described above, various materials employing various transition metal compounds containing a transition metal such as iridium or platinum have been used as highly efficient luminescent materials using phospholuminescence. However, white phosphorescent materials with high emission efficiency and low power consumption have not been developed, making it difficult to achieve a full-color display, which is, in turn, a barrier to the development of phosphorescent full-color display devices.
To address the above-described problems, the development of blue luminescent materials is under way (WO 02/15645 A1, US 2002/0048689 A1 entitled Light-emitting device and iridium complex to Igarashi Tatsuya et al. and published on Apr. 25, 2002). Also, there have been proposed organometallic complexes having bulky functional groups or functional groups having high intensity ligand fields, e.g., a cyano group, introduced thereto to increase a difference between HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) energy levels by transforming the molecular geometry of the oragnometallic complexes. Other materials that have recently been developed include iridium complexes having the general formula of Ir(ppy)2P(ph)3Y, where Y is Cl or CN, as disclosed in US2002/0182441 A1 entitled Organometallic compounds and emission-shifting organic electrophosphorescence to Lamansky, Sergey et al. and published on Dec. 5, 2002, and iridium(III) complexes including a cyclometalating ligand and chelating diphosphine, chlorine and a cyano group, as disclosed in US 2002/0064681 A1 entitled Luminescence device, display apparatus and metal coordination compound to Takiguchi, Takao et al. and published on May 30, 2002.